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Very leaky houses are uncomfortable and have high energy bills, so finding and curing infiltration
problems is a high priority for weatherization operations. The rate of air infiltration in a home depends on many factors, the most important being the size and location of holes in the thermal envelope and the difference in temperature between inside and outside. Warm air inside a dwelling gives rise to stack-effect infiltration as it tries to escape from the top of the envelope, sucking in cold air at the bottom. Wind and leaks in duct systems can also have a major effect on infiltration, but these effects are not usually as constant over the heating season as is stackeffect infiltration, which is at its worst on coldest days.
 Note that in the middle of the heated envelope there is a neutral pressure zone where neither
infiltration nor exfiltration occurs due to stack effect. This explains why caulking and weather stripping in mid-envelope tends to save less energy than careful attention to the bottom
and top of the envelope, where these natural driving forces are greater.
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